In recent years, demand for power MOS semiconductors has been increasing. It thus has become increasingly important to develop N-type silicon single crystals that have low resistivity as substrates for use in these semiconductors.
As a method for manufacturing such an N-type silicon single crystal with low resistivity, a method to pull a single crystal from a silicon melt being doped with an N-type dopant according to a Czochralski method is widely used. In this method, the single crystal is pulled with the N-type dopant added to the silicon melt so as to attain an aimed resistivity. N-type dopants, however, apt to evaporate from the silicon melt, and it is very difficult to control the resistivity of the obtained single crystal to a desired resistivity, especially in pulling a heavy-doped single crystal.
In pulling a heavy-doped N-type single crystal, a dislocation is apt to occur. When a dislocation occurs, a single crystal having the dislocation is re-melted to the silicon melt and re-pulling is conducted as described in paragraph [0015] in Patent Document 1, for example. In this case, the dopant is additionally doped in order to adjust the dopant concentration prior to the re-pulling, since N-type dopants such as phosphorous are apt to form oxides to evaporate from the silicon melt as described above.
The evaporation amount of the dopant during pulling the single crystal can be predicted in a certain degree. In a step to re-melt the single crystal having the dislocation, however, the silicon melt needs to be heated at high temperature and the evaporation amount of the dopant increases due to the high temperature. Therefore, the accuracy of the prediction is decreased.
Accordingly, when the re-melting and the re-pulling is repeated, the dopant concentration in the silicon melt comes to be unknown, and tend to deviate from the aimed concentration considerably. The large deviation from the aimed dopant concentration means that the resistivity of the obtained single crystal considerably deviates from the desired resistivity, which causes a problem that the manufactured single crystal cannot be a product.